GLACE survey: OSIRIS/GTC multi-object spectroscopy of the rich galaxy cluster ZwCl 0024.0+1652 at z~0.4. III. The mass-SFR relation and the quenching of cluster galaxies

TL;DR

Using GLACE MOS data for ZwCl 0024.0+1652 at $z\sim0.4$, the paper quantifies the mass–SFR relation and environmental quenching in a massive cluster. The authors derive SFRs from multiple emission lines, classify galaxies with BPT/WHAN diagnostics, and fit SEDs to obtain $M_\star$ and $D4000$, finding that $\sim34\%$ of SFGs are suppressed in the cluster versus $\sim11\%$ in the field. They show that SFRs of active galaxies do not strongly depend on local density, but the fraction of star-forming galaxies declines toward denser regions and during infall, consistent with a delayed-then-rapid quenching scenario, and that cluster mass is not the primary quenching driver. The results imply prolonged environmental quenching in clusters and lasting impacts on stellar populations, with structure B (infall) contributing prominently to suppression. Overall, the work demonstrates how MOS spectroscopy coupled with SED fitting can reveal the nuanced, time-dependent pathways of galaxy quenching in cluster environments.

Abstract

Galaxy clusters are among the largest and densest structures in the Universe. Their high density generally increases the suppression of star formation, known as quenching, altering galaxy properties. We study the quenching of emission-line galaxies (ELGs) in the rich cluster ZwCl 0024.0+1652 (Cl0024) at redshift $z\sim0.4$, aiming to determine if and how star formation is suppressed. Using multi-object spectroscopy from the GLACE survey, we extracted fluxes and redshifts of [O II]$λ\lambda3727,3729$, $\mathrm{Hβ}$, and [O III]$\lambda5007$ emission lines to derive star formation rates (SFRs) for 173 ELGs. We also performed spectral energy distribution fitting to obtain key evolutionary parameters such as stellar masses ($M_\star$) and the 4000 Å break ($D4000$) index. We derived the $M_\star-\mathrm{SFR}$ relation for 98 star-forming galaxies (SFGs), finding 34.7% exhibit suppressed SFRs in the cluster, compared to 11.0% in the field. While the SFRs show no significant variation with local density, the fraction of SFGs is 1.55 times higher in the cluster outskirts than in intermediate-density regions. The specific SFR decreases strongly with $D4000$ for active SFGs but remains constant for suppressed galaxies. The fraction of suppressed galaxies in the infall region is 2.6 times higher than in the core, especially in the infalling structure B of the cluster. The cluster's total mass does not appear to be a key factor in SFG quenching. Star formation in Cl0024 galaxies is suppressed by the dense cluster environment. This suppression is evident in SFG fractions and parameters tracing long-term evolution, indicating prolonged quenching. The SFGs preferentially reside in low-density regions, while suppressed galaxies dominate the infall region, supporting a 'delayed-then-rapid' quenching scenario.

Figures (24)

• Figure 1:
• Figure 2: Distribution of $M_\star$ computed using CIGALE for 363 galaxies of the cluster (171 ELGs and 192 non-ELGs). The distribution is divided into three segments: low-mass (in blue), intermediate-mass (in green), and high-mass galaxies (in orange). We show the distribution of ELGs with the hatched histograms. In the legend, between parentheses, we include the number of galaxies within each subsample, and in the upper-right corner we show the median of the uncertainty bars of $M_\star$.
• Figure 3: Results of the line fitting process applied on the 161 MOS spectra. (Lower-left panel) The main plot of this figure shows the values of the MOS redshifts calculated using [OII] (yellow), $\mathrm{H\beta}$ (indigo), and [OIII]$\lambda5007$ (magenta) emission lines with respect to $\mathrm{H\alpha}$ redshifts (from SP15 or Cedres2024), in both cases with their respective uncertainties as error bars. The symbols of each emission line are shown in the upper-left region. The green circles show outliers, which are galaxies with MOS redshifts that are different from the $\mathrm{H\alpha}$ redshifts. We linearly fitted each distribution (shown with the coloured dashed lines), taking care to exclude outliers. The dashed black line represents the one-to-one relation. In the lower-right corner we show a zoomed window on the structure A. (Upper-left panel) The histogram shows the distribution of $z_{\mathrm{H\alpha}}$ with the position of each structure (A, B and C) at their corresponding redshift and the number of detections in the upper-right corner (including duplicated $z_{\mathrm{H\alpha}}$ for galaxies 647_b and 657_b that have two MOS spectra each). (Lower-right panel) The histograms show the three distributions of MOS redshifts ([OII] in yellow, $\mathrm{H\beta}$ in indigo, and [OIII]$\lambda5007$ in magenta) with the number of detections at the top. (Upper-right panel) The scatter plot shows how distant are the MOS redshifts from the $\mathrm{H\alpha}$ distribution, using $\log_{10}\left(|z_{\rm MOS}-z_{\rm \mathrm{H\alpha}}|\right)/(1+z_{\rm \mathrm{H\alpha}})$. These values were used to identify the outliers shown with green circles. We show in the lower-right corner the medians of the uncertainty bars of each distribution.
• Figure 4: Distribution of the uncorrected fluxes of the emission lines. MOS emission lines generally have a lower flux than $\mathrm{H\alpha}$. Among them, [OII] fluxes are usually higher, while $\mathrm{H\beta}$ and [OIII]$\lambda5007$ fluxes display a similar distribution.
• Figure 5: Baldwin-Phillips-Terlevich diagram of 62 galaxies of Cl0024. The position in the BPT diagram of each galaxy was calculated using [OIII]$\lambda5007/\mathrm{H\beta}$ and [NII]$\lambda6583/\mathrm{H\alpha}$ ratios when either [OIII]$\lambda5007$ or $\mathrm{H\beta}$ flux was available; in the case where only one of the two fluxes was available, an upper limit of the flux, estimated from the continuum level, was used (arrows on the diagram). The prescriptions of Ho1997, Kewley2001, S06, and Schawinski2007 are shown in the diagram, as well as Kauffmann2003's, which we used as the separation between SFGs and AGN galaxies. Orange circles are SFGs, green squares are AGN galaxies and blue diamonds are composite galaxies. We also show the galaxies initially classified as AGN hosts by SP15 using grey-filled symbols and the two positions in the diagram of the galaxy 647_b with two pink diamonds since it has two MOS spectra with $\mathrm{H\beta}$ detections. The median of the uncertainty bars are shown in the lower-left part of the figure.